This invention relates generally to heat fusing methods and devices, and more particularly, to an improved fusing surface and method which will prevent offsetting of a resin-based powder onto the surface during the fusing operation. As used herein, the fusing surface may be a roll, a flat surface or any other shape suitable for fixing toner or resin-based powder images. The invention is particularly useful in the field of xerography where images are electrostatically formed and developed with resinous powders known as toners, and thereafter fused or fixed onto sheets of paper or other substrates to which the powder images have been transferred. The resin-based powders or toners of this invention are heat softenable, such as those provided by toners which contain thermoplastic resins and used conventionally in a variety of commercially known methods.
In order to fuse images formed of the resinous powders or toners, it is necessary to heat the powder and the substrate to which it is to be fused to a relatively high temperature, generally in excess of about 200.degree. F. This will vary depending upon the softening range of the particular resin used in the toner. Generally, even higher temperatures are contemplated such as approximately 325.degree. F., or higher. It is undesirable, however, to raise the temperature of the substrate substantially higher than 400.degree. F., because of the tendency of the substrate to discolor at such elevated temperatures, particularly when the substrate is paper.
It has long been recognized that one of the fastest and most positive methods of applying heat for fusing the powder image is direct contact of the resin-based powder with a hot surface, such as a heated roll. But, in most instances as the powder image is tackified by heat, part of the image carried by the support material will stick to the surface of the plate or roll so that as the next sheet is advanced on the heated roll, the tackified image, partially removed from the first sheet, will partly transfer to the next sheet and at the same time part of the tackified image from said next sheet would adhere to the heated roll. This process is commonly referred to in the art as "offset", a term now well-known in the art.
The offset of toner onto the heated surface led to the development of improved methods and apparatus for fusing the toner image. These improvements comprised fusing toner images by forwarding the sheet or web of substrate material bearing the image between two rolls at least one of which was heated, the rolls contacting the image being provided with a thin coating of tetrafluoroethylene resin and a silicone oil film to prevent toner offset. The outer surfaces of such rolls have also been fabricated of fluorinated ethylene/propylene or silicone elastomers coated with silicone oil as well as silicone elastomers containing low surface energy fillers such as fluorinated organic polymers, and the like. The tendency of these rolls to pick up the toner generally requires some type of release fluid continuously applied through the surface of the roll to prevent such offset, and commonly known silicone oils, are generally well adapted for this purpose. However, the constant application of the fluid requires a separate fluid reservoir and fluid applying means as well as a metering and control system to maintain the proper amount of fluid on the roll. This requires additional equipment and greater expense in maintaining a supply of fluid. Surfaces without a release fluid are generally inadequate to prevent offset especially when used in high speed xerographic reproduction. It is, therefore, the principal object of this invention to provide a fusing surface and method for rapidly fixing resinous powder images without causing offset.
Another object of the present invention is to provide a silicone rubber fuser surface which makes physical contact with the resinous powder image on the substrate, and which, without the use of a release fluid applicator, provides a layer of release material on the surface of the silicone rubber, thereby preventing offset images thereon.
A further object of this invention is to provide a method of fusing wherein a substrate bearing a resin-based powder image contacts a silicone rubber layer at a temperature sufficient to permit the fusion of the resin-based powder to the substrate, wherein the surface of the silicone rubber layer has an adhesion for the fused resin-based powder which is less than the adhesion which the fused resin-based powder has for the substrate, without the external application of silicone oil or other release fluid to the silicone rubber layer.